Abstract:

Direct methanol fuel cells (DMFC) produce electrical energy directly from chemical energy. They are a promising candidate for power sources of portable devices due to the high energy density of methanol and the quick recharging procedure by fuel insertion. However, the problem areas of the DMFC are the slow electro-oxidation of methanol and the permeated methanol reacting at the cathode. New catalysts are constantly searched but they are often tested only for catalytic activity and the DMFC testing is omitted even though the catalyst layer (CL) structure has a large impact on the performance. Miniaturization of the system is also necessary for portable applications. Silicon etching can be used to fabricate small structures for fuel cells replacing or enhancing the functions of laboratory-scale components.

In the first part of this thesis, new catalysts for the DMFC are studied with the emphasis on the CL structure. Different carbon supports for the anode were studied: standard carbon black and alternative few-walled carbon nanotubes (FWCNT) and graphitized carbon nanofiber (GNF). The alternative supports showed better DMFC performance but their stability was lower than with carbon black. However, the CL formed with GNF showed a very porous structure enhancing the mass transfer, so that higher binder content could be used improving the stability to the level of carbon black and the performance by 30%. The FWCNTs were also investigated as a platform for enzymatic methanol oxidation by studying the electrochemical properties of an immobilized cofactor pyrroloquinoline quinone (PQQ). A large amount of PQQ was adsorbed having a strong redox response and good stability in a wide pH window. For the cathode, a methanol-tolerant, Pt-free nitrogen-doped FWCNTs were tested in an alkaline DMFC as such testing is not often made. Its performance was remarkably 4 times better than with Pt when synthetic air was used as the oxidant.

In the second part of thesis, an integrated gas diffusion layer (GDL) consisting of Si nanoneedles (nanograss) was tested in a micro fuel cell (MFC). The layer functioned properly at low current densities. For high power applications, a standard carbon cloth GDL was tested with the nanograss as a contact surface reducing the resistance between the GDL and the flow field. The use of the nanograss improved the MFC performance and stability. Finally, the MFCs were used as a catalyst testing platform and the results were compared with a similar test in a laboratory-scale DMFC. The results varied showing that the DMFC components also have a large impact on catalyst testing.Suorametanolipolttokennot (SMPK) tuottavat sähköenergiaa suoraan kemiallisesta energiasta. Ne ovat lupaava vaihtoehto kannettavien laitteiden voimanlähteeksi, koska metanolilla on korkea energiatiheys ja lataaminen tapahtuu nopeasti polttoainelisäyksellä. SMPK:lla on kuitenkin rajoitteina metanolin hidas hapetusreaktio ja katodille kulkeutuneen metanolin reagoiminen. Uusia katalyyttejä etsitään jatkuvasti, mutta ne testataan usein vain aktiivisuuden osalta ja SMPK-testaus jätetään pois, vaikka katalyyttikerroksen (KK) rakenne vaikuttaa huomattavasti SMPK:n toimintaan. Systeemin pienentäminen on myös tärkeää kannettavia sovelluksia varten. Piin etsaaminen on tapa tuottaa pieniä rakenteita polttokennoihin korvaamaan ja tehostamaan perinteisten komponenttien toimintaa.